Method and apparatus for controlling power transients in an optical communication system

ABSTRACT

A method and apparatus for controlling transients in an optical signal propagating along an optical fiber path interconnecting a plurality of network elements employs a power threshold to determine an appropriate response given a change in optical signal power.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to lightwavecommunications systems and, more particularly, to a method and apparatusfor controlling power transients in an optical communication system.

[0003] 2. Description of the Related Art

[0004] In the field of fiber optic communications, it has been known forsome time to increase the capacity of an optical communications link bypropagating wavelength-division multiplexed (WDM) optical signals alongoptical fibers. Specifically, a WDM signal is composed of a plurality ofdistinct wavelengths of light, each such wavelength carrying arespective optical information signal, also known as an informationchannel. The number of wavelengths (i.e., information channels) in a WDMsignal is a system parameter and usually ranges from 2 to 128 (in thecase of “dense” WDM, or DWDM).

[0005] As the WDM signal travels through an optical network, itgradually fades and must be amplified at various points along its route.Optical amplifiers are typically provided throughout the optical networkto maintain optical signal levels at their correct power settings.Transients caused by the instantaneous addition or removal of one ormore individual optical information channels by an add/drop multiplexeror other device will affect the power of the WDM signal. Such transientscan cause some channels to have power levels that are too high or lowwith respect to other channels. These transients can cause substantialdegradation in the system's bit error rate and may affect service of theoptical network.

SUMMARY OF THE INVENTION

[0006] The disadvantages associated with the prior art are overcome by amethod and apparatus for controlling transients in an optical signalpropagating along an optical fiber path interconnecting a plurality ofnetwork elements. The present invention detects a change in power of anoptical signal in a first network element that exceeds a threshold. Whenthe power of the optical signal exceeds the threshold, the first networkelement adjusts the power of the optical signal and transmits atransient notification to at least one additional network element. Inone embodiment, the first network element adjusts the power of theoptical signal by controlling at least one gain element disposedtherein, to achieve a desired power per channel of the optical signal.In another embodiment, the present invention establishes a series ofthresholds, and the response of the network elements disposed along anoptical transmission line is dictated by which particular threshold hasbeen exceeded.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] So that the manner in which the above recited features of thepresent invention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings.

[0008] It is to be noted, however, that the appended drawings illustrateonly typical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

[0009]FIG. 1 shows a block diagram of one illustrative lightwavecommunication system embodying the principles of the present invention;

[0010]FIG. 2 shows a block diagram of an illustrative optical amplifierarrangement in a given network element of the lightwave communicationsystem of FIG. 1;

[0011]FIG. 3 is a flow diagram depicting one embodiment of the transientcontrol method of the present invention; and

[0012]FIG. 4 is a flow diagram depicting another embodiment of thetransient control method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013]FIG. 1 depicts a block diagram of an illustrative lightwavecommunication system 100 embodying the principles of the presentinvention. The system comprises an optical transmitter 102, an opticaltransmission line 103, and an optical receiver 106. The opticaltransmitter 102 converts electrical data signals to optical data signalsfor transmission over a series of optical fiber spans 110 in the opticaltransmission line 103 to the optical receiver 106. The optical receiver106 reconverts the optical data signals to electrical signals. Theoptical data signal typically comprises a plurality of wavelengths oflight, each wavelength providing a different optical communicationchannel. For example, the lightwave communication system 100 supportsmany optical channels, illustratively 128 channels, each using adifferent optical carrier wavelength. Optical channels can be modulatedat, for example, 10 Gbps. The carrier wavelengths are illustratively inthe vicinity of 1555 to 1608 nm. These are merely illustrative systemcharacteristics. If desired, more or less channels can be provided,signals may be modulated at a different rate, and a different range ofcarrier wavelengths can be supported.

[0014] The optical transmitter 102 can include laser diodes, each ofwhich supports a channel operating at a different wavelength. If one ormore of these lasers is taken out of service, or if new channels areadded at the optical transmitter 102, the number of wavelengths beingtransmitted across the optical transmission line 103 may changeabruptly. The number of channels being carried by the opticaltransmission line 103 can also change due to unexpected system failures,such as fiber cuts.

[0015] In the present example, the optical transmission line 103includes various network elements, such as multiple stages of repeaters104 and an optical add/drop multiplexer (OADM) 108. In general, opticaltransmission line 103 could be any type of simple or complex arrangementof components. The repeaters 104 and OADM 108 are separated by the spansof optical fiber 110. Fiber spans may be on the order of 40-120 km inlength for long-haul networks, or may be any other suitable length foruse in signal transmission in a lightwave communication system.Repeaters 104 include gain elements (an example is shown in FIG. 2) foramplifying the optical data signal as it travels along the opticaltransmission line 103. The OADM 108 can be used to separate channels atcertain wavelengths from the optical data signal. The separated channelsmay be provided to another network (not shown). In addition, the OADM108 can be used to add channels at certain wavelengths to the opticaldata signal. The operation of add/drop multiplexers, such as OADM 108,is well known in the art. In some instances, the number of channelsadded to, or dropped from, the optical transmission line 103 by the OADM108 can change abruptly.

[0016] As known by those skilled in the art, the total optical power ofa signal propagating in a given fiber span 110 is the sum of the powersof the individual wavelength components. Thus, if a given fiber span 110is initially carrying ten signal channels, the total input power to anetwork element coupled to the fiber span is relatively constant. Ifnine of the ten channels are suddenly dropped either by the OADM 108 orthe transmitter 102, the total power input to a network element coupledto the fiber span is now one-tenth its original level. When this lowerpower signal is coupled to a saturated optical amplifier in the networkelement, the output power of the optical amplifier will be nearlyunchanged. Thus, the single remaining channel will now have all of thepower that was once distributed among ten channels, and the lightwavecommunication system may not be able to achieve good performance withthe surviving channel having such a high power. In addition, since theoutput power of the network element will be substantially unchanged,downstream network elements will not be able to detect the transientimmediately.

[0017] Furthermore, if the original channel population is, for example,80 channels, and 78 of those channels are suddenly removed, theremaining two channels will have very high power as discussed above. Theremaining two channels will also have very different power levels due tochannel-to-channel and channel-to-amplifier interactions. Theseinteractions are properly balanced for 80 channels, but now theamplifier settings must be adjusted for 2 channels.

[0018]FIG. 2 shows a block diagram of an illustrative optical amplifierarrangement in a network element embodying the transient controlprinciples of the present invention. More specifically, a given networkelement comprises an optical amplifier 204 disposed along an opticalfiber path 203. Optical amplifier 204 receives the optical signal froman upstream network element and supplies and amplified optical signaldownstream along optical fiber path 203. For uniformity and ease ofunderstanding in the following description, optical amplifier 204 iscontemplated to be a rare earth-doped optical fiber amplifier, such asan erbium-doped fiber amplifier (EDFA), though other amplifiers, such asRaman amplifiers may also be employed.

[0019] In order to provide an amplifying effect, the optical amplifier204 is “pumped” with luminous energy using techniques known in the art(internal pump sources not shown). The luminous energy, also referred toas pump light, generally has a shorter wavelength than any of thewavelengths in the optical signal. In addition, Raman pumps 206 and 208can be used to further amplify the optical signal in optical fiber path203 via Stimulated Raman Scattering (SRS). Specifically, Raman pump 208provides counter-propagating pump light to the optical fiber 203. Inthis sense, Raman pump 208 serves as a pre-amplifier to the opticalamplifier 204. Raman pump 206 provides co-propagating pump light to theoptical fiber 203 to provide further downstream amplification of theoptical signal. Raman pumps 206 and 208, as well as pumps in the opticalamplifier 204 (not shown), can be semiconductor laser pump assemblies,such as laser diode pumps or any other suitable pump sources well knownin the art. Optical couplers 207 are used to couple the pump lightemitted by Raman pumps 206 and 208 to the optical fiber path 203. Theuse of optical couplers 207 for this purpose is also well known to thoseskilled in the art. Although the present invention is described withrespect to a hybrid Raman amplifier-EDFA arrangement, other knownarrangements of optical amplifiers and/or optical pumps can also be usedwithout departing from the spirit and scope of the present invention.

[0020] An optical spectrum analyzer (OSA) 202 is coupled to the opticalfiber path 203 at the input and output of the optical amplifier 204 viamonitor ports 209. The output of the OSA 202 is coupled to controller220. In general, the OSA 202 can monitor points along the optical fiber203 at the inputs and outputs of various optical components comprising anetwork element. The OSA 202 is capable of monitoring severalcharacteristics of the optical signal, such as signal-to-noise ratio,channel population, power distribution among the various channels, etc.The OSA 202 operates by periodically monitoring the optical signalcharacteristics (known as “scanning”), and reporting the characteristicsto the controller 220 for a power adjustment, if necessary.

[0021] Additional monitoring taps 211 are coupled to the optical fiberpath 203 for determining the input and output power levels of theoptical signal at optical amplifier 204. Specifically, monitoring taps211 are coupled to photodetectors 210. Photodetectors 210 could be anysuitable device known to those skilled in the art (e.g., a photodiode)for detecting optical energy and converting the optical energy to anelectrical signal. The electrical signals from the photodetectors 210are processed through power monitors 212, which relate the photocurrentof their respective photodetector 210 to the power level of the opticalsignal in its electrical form. Suitable circuitry for each of the powermonitors 212 is also well known in the art. The outputs of the powermonitors 212 are also coupled to the controller 220.

[0022] Controller 220 is further coupled to a pump controller/driver 216and, optionally, a supervisory control circuit 218. The pumpcontroller/driver 216 adjusts the bias circuitry (not shown) of theoptical amplifier 204 and Raman pumps 206 and 208 in order to achieve adesired power per channel of the optical signal. Supervisory controlcircuit 218 is optionally used to carry out specified control andmanagement functions. In particular, supervisory control circuit 218transmits a supervisory signal along optical fiber path 203 to the nextnetwork element on the optical transmission line. In another embodiment,supervisory control circuit 218 transmits the supervisory signal alonganother optical fiber path (not shown). As described in more detailbelow, the present invention optionally employs the supervisory signalto notify network elements downstream from a transient of the occurrenceof the transient. This allows downstream network elements to adjusttheir power levels.

[0023] Operation of one embodiment of the present invention is bestunderstood by simultaneous reference to FIGS. 2 and 3. FIG. 3 is a flowdiagram illustrating one embodiment of a transient control method 300 inaccordance with the present invention. The transient control method 300is executed by the controller 220 in a given network element. Controller220 can comprise a processor, a memory, support circuits, and otherknown processing components for executing the method 300 of the presentinvention.

[0024] At step 302, the transient control method 300 starts. At step304, the controller 220 receives the current steady state input powerP_(BASE) supplied to the network element from an optical fiber span 110(FIG. 1). The controller 220 receives the power reading from the powermonitor 212 coupled to the input of optical amplifier 204. At step 306,controller 220 defines a first transient threshold P₁ such that inputpower exceeding P₁ requires a power adjustment in the network element,as well as power adjustments in one or more downstream network elementsalong the optical transmission line. In one embodiment, P₁ is selectedto be the threshold level of input power change that requires animmediate adjustment in gain element settings in both the networkelement detecting the transients and one or more downstream networkelements. P₁ can be a single value or be defined as a range of powerlevels.

[0025] At step 308, the power monitor 212 at the input of the opticalamplifier 204 detects when the input power to the network elementexceeds the first transient threshold P₁. As described above, a changein input power at a given network element can result from an abruptchange in channel population in the optical fiber. Although the presentinvention is described as monitoring input power to a network element ingeneral, those skilled in the art understand that the present inventioncan monitor input power to any of the various components comprising anetwork element, such as an optical amplifier. The method 300 thenproceeds to step 310, where the various gain elements in the networkelement are adjusted to achieve a desired power per channel in theoptical signal. In the present embodiment, gain elements include opticalamplifier 204 and Raman pumps 206 and 208.

[0026] At step 312, the network element transmits a transientnotification to downstream network elements. As described above, incertain instances, gain elements within the network element may becomepartially or fully saturated such that their output power remains thesame regardless of the change in input power. In this case, downstreamnetwork elements may not be able to detect the power transient withoutdetermining the channel population. Thus, the present inventionadvantageously transmits a transient notification to at least onedownstream network element. In one embodiment, the network elementemploys a supervisory signal controlled by the supervisory controlcircuitry 218. In this embodiment, notification step 312 comprisessetting a transient indicator in the SONET overhead of the supervisorysignal. In another embodiment, the notification step 312 comprisessupplying an additional optical signal, independent of the presence ofthe supervisory signal, for transmitting the transient indicatordownstream. In yet another embodiment, the controller 220 causes thegain elements in the network element to “pulse” the optical signal in away that the downstream network elements will detect the transient.

[0027] In any case, at step 314, the downstream network elements performpower adjustment in response to the notification of the transient.Again, this power adjustment can comprise adjusting gain elements toachieve a desired power per channel and notifying additional downstreamnetwork elements of the transient. At step 316, the method 300 ends.

[0028]FIG. 4 is a flow diagram of another embodiment a transient controlmethod 400 of the present invention. Again, FIG. 4 should besimultaneously referenced with FIG. 2. At step 402, the method 400begins. At step 404, the controller 220 receives the current steadystate input power P_(BASE) substantially as described above with respectto FIG. 3. In the present embodiment, the controller 220 establishes afirst transient threshold P₁ and a second transient threshold P₂ at step406. In one example, P₂ is defined as P_(BASE)±1 dB, and P₁ is definedas P_(BASE)±3 dB. That is, input power over the first power threshold(or range) P₁ would require a power adjustment in the network element,as well as power adjustments in one or more downstream network elementsalong the optical transmission line. Input power over the secondthreshold (or range) P₂ would be a moderate power excursion, requiringonly local power adjustment within the network element. The values forP₁ and P₂ are exemplary, and can be modified as desired for a particularlightwave communication system.

[0029] At step 408, the power monitor 212 coupled to the input of theoptical amplifier 204 detects a change in the input power (ΔP) to thenetwork element. Again, the input power can also be monitored at any ofthe various components comprising a network element. At step 410, thecontroller 220 determines whether or not the change in input power isgreater than the second power threshold P₂. If the change in input poweris less than the second power threshold P₂, controller 220 takes noimmediate action to adjust the optical signal power at step 412. If aregularly scheduled scan by OSA 202 determines that there is a change ininput channel count or power distribution, the normal transmissioncontrol algorithms are used to adjust the optical signal power.

[0030] At step 416, the controller 220 determines if the change in inputpower is between the second power threshold P₂ and the first powerthreshold P₁. If so, the method 400 proceeds to step 416, where thecontroller 220 causes the pump controller/driver 216 to perform a pumppower adjustment in the optical amplifier 204 and the Raman pumps 206and 208. Specifically, the power monitor 212 coupled to the input of theoptical amplifier 204 reads the new input power, and the optical signaloutput power of the optical amplifier 204 (determined by the powermonitor 212 coupled at the output) is adjusted to maintain the desiredpower per channel. The second transient threshold P₂ is selected to bethe threshold level of input power change that requires immediateadjustment of pump and amplifier settings in the network element,instead of waiting for the periodic adjustment that would occur as aresult of a scan by the OSA 202.

[0031] If the input power change is greater than the first powerthreshold P₁, the method 400 proceeds to step 418. Specifically, the OSA202 determines the channel population of the optical signal. At step420, the supervisory control circuitry 218 notifies the downstreamnetwork elements of the detected transient substantially as describedabove with respect to step 312 of the method 300. Of course othernotification methods can also be used. At step 422, new pump signalpowers for the Raman pumps 206 and 208 are determined. In oneembodiment, the pump power of Raman pumps 206 and 208 is adjusted basedon extrapolation from existing pump settings for the last non-zerochannel population using the current channel population, rather thanbased on an attempt to correct measured power levels. In addition, thepump signal power in the optical amplifier 204 is also adjusted at step422.

[0032] At optional step 424, the OSA 202 will determine the channelpopulation of the optical signal for a second time. If the channelpopulation is unchanged, the method proceeds to end step 426. If thechannel population has changed, pump re-adjustment is performedsubstantially as described above with respect to step 422. The method400 then proceeds to end step 426.

[0033] In other embodiments of the invention, more than two thresholdlevels are used (i.e., P₁, P₂, . . . P_(N)). In these embodiments, thevarious threshold levels are preferably selected with respective changesin input power levels. The response of a given network element is basedupon which threshold has been exceeded. For example, in addition to thetwo thresholds described above, there could be a third threshold inbetween the first and second thresholds. If the input power to a networkelement exceeded this third threshold (but is less than the higher firstthreshold), then the network element could adjust the power of theoptical signal locally, and transmit a transient notification to asubset of network elements less than that which would be notified if thepower level exceeded the higher first threshold. That is, the number ofadditional network elements notified of the transient can vary basedupon a particular threshold that has been exceeded. In this embodiment,the method 400 of FIG. 4 is modified to accommodate additional thresholdlevel comparisons and additional control actions.

[0034] Methods 300 and 400, along with the alternative embodiments ofthose methods, are performed by a given network element in response to atransient input to the network element, or in response to a transientnotification sent from another network element. When a given networkelement receives notification that a transient occurred upstream, thatnetwork element executes steps 418, 420, 422, and optionally 424 ofmethod 400. In addition, those skilled in the art will appreciate thatsteps 418, 420, 422, and optional step 424 may be performed in adifferent order, and/or simultaneously as desired.

[0035] As previously described, the principles of the present inventioncan also be advantageously used to control optical signal power suppliedby other optical components even though the above embodiments weredescribed only in the context of optical amplifiers. For example, thepresent invention can be used to control power levels of optical pumpsin dispersion compensation modules or lasers in optical transmitters.Thus, while the foregoing is directed to the preferred embodiment of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

1. A method, comprising: adjusting a power level of an optical signal ina first network element, in response to the optical signal exhibitingpower exceeding a first threshold; and transmitting a transientnotification to at least one additional network element, in response tothe optical signal exhibiting power exceeding the first threshold. 2.The method of claim 1, further comprising: causing the power level of anoptical signal in the at least one additional network element to beadjusted, in response to the transient notification.
 3. The method ofclaim 1, further comprising the step of adjusting a power level of theoptical signal in the first network element, in response to the opticalsignal exhibiting power exceeding a second threshold, the secondthreshold being less than the first threshold.
 4. The method of claim 1,wherein the step of adjusting power comprises adjusting output powersupplied by at least one gain element to achieve a desired power peroptical channel of the optical signal.
 5. The method of claim 4, whereinthe step of adjusting output power supplied by at least one gain elementcomprises: determining a current channel population of the opticalsignal; adjusting pump power supplied by at least one pump source; andadjusting gain supplied by at least one optical amplifier.
 6. The methodof claim 1, wherein the first threshold comprises a steady state powerlevel in the first network element plus or minus a predetermined amount,the predetermined amount being selected such that a power level of theoptical signal exceeding the first threshold requires adjustment of thepower of the optical signal at more than one network element.
 7. Themethod of claim 3, wherein the second threshold comprises a steady statepower level in the first network element plus or minus a predeterminedamount, the predetermined amount being selected such that a power levelof the optical signal exceeding the second threshold requires adjustmentof the power of the optical signal at a single network element.
 8. Anetwork element adapted for use in an optical transmission system, thenetwork element comprising: at least one gain element, for providing anoptical signal to an optical transmission line; and a controller, foradjusting the output power of the at least one gain element to achieve adesired power per channel of the optical signal, and transmitting atransient notification to at least one additional network element, inresponse to the optical signal exhibiting power exceeding a firstthreshold.
 9. The network element of claim 8, wherein the controlleradjusts only the output power of the at least one gain element, inresponse to the optical signal exhibiting power exceeding a secondthreshold, the second threshold being greater than the first threshold.10. The network element of claim 8, wherein the at least one gainelement comprises at least one of an optical amplifier and an opticalpump source.
 11. The network element of claim 8, wherein the firstthreshold comprises a steady state power level plus or minus apredetermined amount, the predetermined amount being selected such thata power level of the optical signal exceeding the first thresholdrequires adjustment of the power of the optical signal at more than onenetwork element disposed along the optical transmission line.
 12. Thenetwork element of claim 9, wherein the second threshold comprises asteady state power level plus or minus a predetermined amount, thepredetermined amount being selected such that a power level of theoptical signal exceeding the second threshold requires adjustment of thepower of the optical signal at a single network element disposed alongthe optical transmission line.
 13. In a lightwave communication systemhaving a plurality of network elements for supplying an optical signaladapted for transmission in an optical fiber path, an apparatus forcontrolling power of an optical signal propagating in the optical fiberpath comprising: means for detecting a transient that causes power of anoptical signal in a first network element to exceed a first threshold;and a first transient control circuit, responsive to a detectedtransient, for adjusting the power of the optical signal in the firstnetwork element and transmitting a transient notification to at leastone additional network element.
 14. The apparatus of claim 13, furthercomprising: means for detecting a transient notification in the at leastone additional network element; and at least one additional transientcontrol circuit, responsive to a detected transient notification, foradjusting the power of the optical signal in the at least one additionalnetwork element.
 15. The apparatus of claim 13, wherein the firsttransient control circuit adjusts the power of the optical signal byadjusting output power of at least one gain element in the first networkelement to achieve a desired power per channel of the optical signal.16. The apparatus of claim 15, wherein the at least one gain elementcomprises at least one of an optical amplifier and an optical pumpsource.
 17. The apparatus of claim 13, wherein the first thresholdcomprises a steady state power level plus in the first network elementplus or minus a predetermined amount, the predetermined amount beingselected such that a power level of the optical signal exceeding thefirst threshold requires adjustment of the power of the optical signalat more than one network element.
 18. The apparatus of claim 13, whereinthe first transient control circuit adjusts only the power of theoptical signal in the first network element, in response to the opticalsignal exhibiting power exceeding a second threshold, the secondthreshold being less than the first threshold.
 19. The apparatus ofclaim 18, wherein the second threshold comprises a steady state powerlevel plus or minus a predetermined amount, the predetermined amountbeing selected such that a power level of the optical signal exceedingthe second threshold requires adjustment of the power of the opticalsignal at a single network element.